1. Field of the Invention
The present invention relates generally to a hybrid electric vehicle (HEV), and specifically to a method and an apparatus for controlling charging and/or discharging of a HEV battery.
2. Discussion of the Prior Art
The need to reduce fossil fuel consumption and emissions in automobiles and other vehicles predominately powered by internal combustion engines (ICEs) is well known. Vehicles powered by electric motors attempt to address these needs. Another alternative solution is to combine a smaller ICE with electric motors into one vehicle. Such vehicles combine the advantages of an ICE vehicle and an electric vehicle and are typically called hybrid electric vehicles (HEVs). See generally, U.S. Pat. No. 5,343,970 to Severinsky.
The HEV is described in a variety of configurations. Many HEV patents disclose systems where an operator is required to select between electric and internal combustion operation. In other configurations, the electric motor drives one set of wheels and the ICE drives a different set.
Other, more useful, configurations have developed. For example, a series hybrid electric vehicle (SHEV) configuration is a vehicle with an engine (most typically an ICE) connected to an electric motor called a generator. The generator, in turn, provides electricity to a battery and another motor, called a traction motor. In the SHEV, the traction motor is the sole source of wheel torque. There is no mechanical connection between the engine and the drive wheels. A parallel hybrid electrical vehicle (PHEV) configuration has an engine (most typically an ICE) and an electric motor that work together in varying degrees to provide the necessary wheel torque to drive the vehicle. Additionally, in the PHEV configuration, the motor can be used as a generator to charge the battery from the power produced by the ICE.
A parallel/series hybrid electric vehicle (PSHEV) has characteristics of both PHEV and SHEV configurations and is sometimes referred to as a xe2x80x9cpowersplitxe2x80x9d configuration. In one of several types of PSHEV configurations, the ICE is mechanically coupled to two electric motors in a planetary gear-set transaxle. A first electric motor, the generator, is connected to a sun gear. The ICE is connected to a carrier. A second electric motor, a traction motor, is connected to a ring (output) gear via additional gearing in a transaxle. Engine torque can power the generator to charge the battery. The generator can also contribute to the necessary wheel (output shaft) torque if the system has a one-way clutch. The traction motor is used to contribute wheel torque and to recover braking energy to charge the battery. In this configuration, the generator can selectively provide a reaction torque that may be used to control engine speed. In fact, the engine, generator motor and traction motor can provide a continuous variable transmission (CVT) effect. Further, the HEV presents an opportunity to better control engine idle speed over conventional vehicles by using the generator to control engine speed.
The desirability of combining an ICE with electric motors is clear. There is great potential for reducing vehicle fuel consumption and emissions with no appreciable loss of vehicle performance or drive-ability. The HEV allows the use of smaller engines, regenerative braking, electric boost, and even operating the vehicle with the engine shutdown. Nevertheless, new ways must be developed to optimize the HEV""s potential benefits.
One such area of HEV development is in the area of methods and systems for controlling the charging and/or discharging of the HEV battery used to store electricity. As explained above, one advantage of the HEV is the ability to limit ICE usage by operating for periods of time using partially or primarily electric power. Quite obviously, if the ICE is to be substantially or totally shutdown, there must be some mechanism for storing electricity to permit the vehicle to operate under electric power during such periods. A rechargeable battery is a conventional mechanism for charge storage that has attained widespread use.
Typically, the HEV battery is constructed from cells or modules that are coupled together to provide the desired voltage. For example, the common 9-volt household battery is in fact a combination of six 1.5-volt cells or modules coupled together in series. HEV batteries are constructed in a similar manner, and may include upwards of 200 modules.
While HEV batteries may be rechargeable so as to extend their operational life, such rechargeable batteries do not have an unlimited life. Moreover, the life of the rechargeable HEV battery may be foreshortened if proper care is not taken in its use and maintenance.
For example, variations may arise between the performance characteristics of the cells or modules in the HEV battery as a consequence of manufacturing or material tolerances, or flaws that may develop with the passage of time. Such variations can cause the effected module to accept less charge, to store less charge, and to provide less charge. Such variations may also lead to the failure of the module if the module""s diminished capacity is exceeded. Consequently, it is known in the art to monitor the charge of the modules and to balance the charge on the modules automatically to prevent overcharging. See, for example, U.S. Pat. Nos. 5,969,624 and 4,313,080.
Maintaining proper charge balance, while helpful to prolong battery life, is not a complete answer to all of the problems, which may cause performance degradation. For example, rechargeable batteries are susceptible to a phenomenon commonly referred to as memory effect. Memory effect occurs when a module or cell is discharged and recharged only partially (shallowly) over several cycles. Eventually, the module will not accept the same charge it did initially.
While memory effect may be detected by the on-board electronics responsible for charge balancing, the onus is commonly put on the user to perform the deep discharge necessary to limit or reverse the memory effect on the effected module. The user may have to connect a separate discharge device to the affected module, or couple a separate on-board discharging circuit to the affected module. However, user intervention may be intentionally or unintentionally delayed such that remedial measures come too late to optimally limit the memory effect.
Moreover, there are other disadvantageous events that may affect battery performance and life that conventionally are not even detected.
Accordingly, an object of the present invention is to provide a battery control method and apparatus for a hybrid electric vehicle (HEV) that detects battery life and/or performance degrading events and takes appropriate remedial measures without requiring user intervention.
A further object of the present invention is to provide an HEV battery control method and apparatus that detects battery life and/or performance degrading events and takes appropriate remedial measures while limiting the effects of such remedial measures on vehicle performance.
An additional object of the present invention is to provide specific strategies for controlling charging and/or discharging of an HEV battery in response to battery life and/or performance degrading events while limiting effects on vehicle performance, thereby rendering the battery control method and apparatus transparent to the user.